1. Field of the Invention
The present invention relates to an optical transmission system, an optical receiver for use in the system, and its gain adjustment method.
2. Description of the Related Art
An optical transmission method is usable in order to expand a service area of a mobile communication system. In other words, distributing radio frequency signals (hereinafter, referred to as radio signals) eliminated from a base station of the mobile communication system to a remote place via an optical fiber enables expanding its service providing area. Such a system has been known as a radio-over-fiber (ROF) system. The optical fiber being small in transmission loss, the ROF system can dispose a large number of remote units to re-emit radio signals over a range of several km.
FIG. 1 is a block diagram illustrating an existing ROF system. In FIG. 1, a master unit 200 and a remote unit power supply device 300 are disposed near by a base station 100. The master unit 200 sends and receives radio signals in uplink/downlink to and from the base station 100. Monitor control signals which controls the master unit 200 and the power supply device 300 are also transmitted and received between the base station 100 and the master unit 200.
The master unit 200 is connected to a plurality of remote units 500 (#1-#n) via optical fiber cables 400. The optical fibers 400 transmit radio signals converted into optical signals. The ROF system includes the optical fibers 400 for each uplink/downlink sometimes, and conversely, it shares a single optical fiber 400 in the uplink/downlink in the use of a wavelength multiplex technique sometimes.
Remote units 500 are supplied power from the remote unit power supply devices 300 disposed near by the master unit 200 through power cables 600. In recent years, since the number of remote units has become several hundreds, the power consumption of each remote unit 500 should be suppressed. If the power consumption at each remote unit 500 is large, the power supply device 300 must be large in capacity, increasing the cost of the equipment. In addition to this, the operating cost increases, caused by the increase in power consumption.
The smaller the power consumption for each remote unit 500 is, the larger the operating margin of the power supply device 300 becomes, so that the ROF system easily copes with an increase in the number of remote units 500. Thus it is important to suppress the power consumption of each remote unit 500 as much as possible.
Meanwhile, transmission paths of the optical fibers 400 have great variety in length, and transmission losses of optical signals vary for each remote unit 500. Therefore, optical output levels of optical transmitters equipped for the remote units 500, and light reception gains of optical receivers, etc., are automatically adjusted on the basis of the received light levels.
The transmission loss of the optical fiber cable 400 which has constructed once hardly varies. Therefore, in transmitting the optical signals, the system operates an output automatic adjustment function to quit a light reception gain adjustment function. The system, in receiving the optical signals, operates the light reception gain adjustment function to quit the output automatic adjustment function. Like this, a technique which operates either a transmission power adjustment function or a reception gain adjustment function in response to the transmission and reception of the optical signals to decrease the power consumption has been proposed (for example, JP-A H9-55705 (KOKAI)